The production of petroleum from new reservoirs has led in recent years to operations in frigid offshore environments where large bodies of moving ice are found. These large moving bodies of ice can severely damage offshore structures such as production platforms and underwater pipelines which lie in their paths.
An example of such an area is off the north coast of Alaska in the Beaufort Sea. With the onset of winter, the sea water near the coastline begins to freeze over. This results in the formation of a relatively smooth and continuous sheet of ice called fast ice which extends seaward from the shore to points which lie over water approximately 60 feet deep. The name fast ice implies that this sheet of ice is held fast to the land and does not move. Fast ice can, however, be moved by natural forces such as currents, tides, and temperature changes, with the rate of movement being generally dependent on the thickness of the ice.
When set in motion, fast ice poses a threat to offshore operations. When the ice comes into direct contact with an offshore drilling structure such as a production platform, large forces may develop. These forces cause the ice sheet to break and pile up directly against the offshore structure, forming a rubble field. As the rubble field grows and continues to be pressed against the structure, the forces can increase until the structure is seriously damaged.
Although it is subject to movement, fast ice is relatively stable during the winter. However, the fast ice sheet breaks up during the summer, resulting in the formation of many individual floating bodies of ice which are free to move about under the influence of winds and currents. These moving bodies of ice pose another threat to offshore operations.
Seaward of the fast ice zone is pack ice. Unlike fast ice, pack ice is discontinuous, rugged, and highly mobile. As pack ice moves, local areas of tension and compression develop, causing the ice to break and pile up. As a result, open leads and pressure ridges are formed.
Pressure ridges form in areas of pack ice which experience large compressive forces. The ice breaks and piles up, concentrating large masses of ice into relatively small areas. Pressure ridges extend well above and below the surrounding ice, and some are so large that they are able to survive the summer and become multiyear ice features.
During the winter season, many pressure ridges are embedded in the pack ice and move along with it, threatening any structure in their path. During the summer, pressure ridges can be blown toward shore, where they threaten structures which lie in shallow waters.
Finally, other moving bodies of ice such as glacial icebergs and floebergs also pose serious threats to offshore operations.
Numerous approaches have been suggested for protecting offshore structures from large moving bodies of ice. For example, U.S. Pat. No. 3,436,920 (Blenkarn et al.) discloses the use of a fence-like barrier which is erected around an offshore structure. Methods such as this have serious drawbacks due to both the time and expense involved in their construction and the expense of the materials. The general lack of availability of construction materials in arctic regions usually means they have to be transported great distances. The structures must then be built, placed in position and anchored to the sea floor.
U.S. Pat. No. 4,523,879 (Finucane et al.) avoids many of the drawbacks associated with the use of barriers which must be assembled from materials not readily available in arctic regions. It calls for the use of ice made by spraying water drawn from the surrounding sea outward and into the air about the offshore structure to form a spray ice barrier about the structure. Although this approach is highly effective for protecting temporary exploration drilling structures, it has its drawbacks where year-round protection of permanent production structures is required. Spray ice barriers tend to deteriorate during summer due to warm ambient temperatures, and due to wave and current erosion as well, and may disappear before the end of the summer. Other ice structures which have been suggested, such as the ice rubble generator - a steel frame or sunken barge designed to trigger pileup of moving ice - tend to share this drawback to varying extents.
More permanent protective structures may be formed using gravel or soil. U.S. Pat. No. 3,990,252 (Louden) describes (1) admixing a slurry of dredgings from the ocean floor with hydraulic cement and soluble alkali silicate, and then (2) placing this slurry between forms (or in sandbags or cans) to form an artificial island perimeter. Although this approach both avoids the warm weather deterioration drawbacks associated with the ice barrier approaches and utilizes readily available seafloor dredgings, it does require the construction of forms (or the use of sandbags or cans) and consequently suffers from the material supply and expense drawbacks described above.
Some protective structures have been constructed using soil materials without forms, bags, or cans. The Seawater Treatment Plant installed by the Prudhoe Bay unit at a 13 foot water depth location is protected on three sides by a U-shaped gravel berm which is contiguous with the structure and which has an 18 foot freeboard. Good quality gravel and well-controlled construction techniques were used to achieve the berm strength required to resist expected ice forces. In order to obtain the correct quality materials, however, the gravel had to be brought to the site from an onshore gravel pit--an approach which would be very expensive for a large structure further offshore. Locally dredged materials do not generally provide adequate strength for such an approach. Even with the superior quality materials used at the Seawater Treatment Plant site, some significant maintenance has been required to repair eroded parts of the berm, especially in its above-water portion. In addition, it is readily apparent that an excessive amount of unstrengthened material would be necessary to construct such protective berm in deeper water. Furthermore, as the Seawater Treatment Plant berm is contiguous with the structure, harmful ice forces may be readily transmitted through the berm to the structure. The Seawater Treatment Plant avoids large moving bodies of ice capable of imparting serious ice forces largely by virtue of its location in shallow water.
In addition to the ice forces described above, unprotected offshore structures are also subject to other natural damaging forces, including the large waves and strong currents frequently encountered in deep water locations.
Consequently, there is still needed an economically constructs means for protecting offshore structures from moving bodies of ice and other natural forces. Such means would make it much more economical to produce oil and gas from offshore reservoirs in the arctic regions.